System and method for CMOS image sensing

ABSTRACT

A system and method for sensing image on CMOS. According to an embodiment, the present invention provide a CMOS image sensing pixel. The pixel includes an n-type substrate, which includes a first width and a first thickness. The pixel also includes a p-type epitaxy layer overlying the n-type substrate. The p-type epitaxy layer includes a second width and a second thickness. The second width is associated with one or more characteristics of a colored light. The pixel additionally includes an n-type layer overlying the p-type epitaxy layer. The n-type layer is associated with a third width and a third thickness. Additionally, the pixel includes an pn junction formed between the p-type epitaxy layer and the n-type layer. Moreover, the pixel includes a control circuit being coupled to the CMOS image sensing pixel.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.200710094550, filed Dec. 13, 2007, commonly assigned and herebyincorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

This invention is directed to imaging techniques. More particularly, theinvention provides a method and system for capturing images on CMOScolor sensors. Merely by way of example, the invention has been used tocapture true color information on each pixel of an N-type substrate CMOSsensor. But it would be recognized that the invention has a much broaderrange of applicability.

To capture vivid imageries have been long been an endeavor thatpersisted as long as the human race itself. As early as the Stone Age,people tried to capture of what they see with cave drawings. Over thethousands of years, artists developed techniques for capture images withpaint brushes and canvases. With oil paintings, artists have been ableto capture real world images with accuracy, but the fidelity ofpaintings is no match to photography.

In 1826, a French inventor Nicéphore Niépce produced the firstphotographic image on polished pewter plate covered with a petroleumderivative. Since then, the technique for photographic imaging evolved.Better techniques and equipment improved image quality over the nexthundred of years. Over the last fifty years, techniques for colorphotography improved and consummated. In the last decade, with theintroduction of the first commercially available digital camera by Kodakin 1990, a new type of image capturing technique, digital imaging, hasrapidly become a popular way for capturing images.

For digital imaging, the image sensor (or digital equivalent of filmnegatives) is one of the most important components for digital imagingdevices, such as digital cameras, camera phones, etc. For a long time,image sensors have been based on charge-coupled device (CCD) technologythat has been developed by George Smith and Willard Boyle at Bell Labs.In the past, CCD based imaging devices dominated. Recently, CMOS basedimage sensors have become more popular.

The CMOS image sensor technology typically includes millions of sensorpixels (light sensing units), each of the sensor pixel includes two tofour transistors and a photodiode. Typically, conventional techniquesfor CMOS image sensing use one np junction, with very shallow p+ layerapplied on top of N region to reduce noise and enhance blue response inimage capturing process. In a way, the CMOS sensor unit 10 works in asimilar manner as a capacitor. The more charge stored in the electrode,the higher the voltage drop across the depletion region of the CMOS.Light, which is a energy source, generates free carriers. The freecarriers under electric field run towards the N type region of the CMOSsensor and neutralized the charge and reduce potential. The voltagedifference before and after the integration of energy provides a signallevel. The signal level is then used as a reading as the amount of lightbeing detected and used for forming an image.

Depending upon applications, CMOS sensor often have advantages over CCDsensors. For example, compared to CCD image sensors, CMOS sensorsusually provide lower costs and longer battery life. As a result, CMOSis often preferred for portable imaging devices such as camera phone andpoint and shoot cameras. At the high end, CCD image sensors are oftenbehind CMOS images sensors in terms of noise level and sensitivity.Because of various advantages of CMOS image sensors, the technologiesfor CMOS image sensors have been rapidly developing. The resolution ofCMOS image sensor has been increasing as pixel size shrink together withthe MOS transistor channel length. While the increase resolution ofimage sensors often improve image resolution, the decreased pixel sizeand increase noise level have become obstacles for improved imagequality. Various techniques, such as improvements on structure andcircuitry, have been developed to improve performance at level. Forexample, various systems and methods have been developed to providecolor separation. In the past, three major approaches have been used toprovide color separation: color filter, stack junction photo diode, andjunction separation. Unfortunately, the abovementioned techniques forimage sensing and color separation are often inadequate. These and otherlimitations of the conventional techniques have been overcome, at leastin part, by the invention that has been fully described below.

Therefore, it is desirable to have an improved method and system for aCMOS image sensing device.

BRIEF SUMMARY OF THE INVENTION

This invention is directed to imaging techniques. More particularly, theinvention provides a method and system for capturing images on CMOScolor sensors. Merely by way of example, the invention has been used tocapture true color information on each pixel of an N-type substrate CMOSsensor. But it would be recognized that the invention has a much broaderrange of applicability.

According to an embodiment, the present invention provide a CMOS imagesensing pixel. The pixel includes an n-type substrate, which includes afirst width and a first thickness. The pixel also includes a p-typeepitaxy layer overlying the n-type substrate. The p-type epitaxy layerincludes a second width and a second thickness. The second width isassociated with one or more characteristics of a colored light. Thepixel additionally includes an n-type layer overlying the p-type epitaxylayer. The n-type layer is associated with a third width and a thirdthickness. Additionally, the pixel includes a pn junction formed betweenthe p-type epitaxy layer and the n-type layer. Moreover, the pixelincludes a control circuit being coupled to the CMOS image sensingpixel.

According to another embodiment, the present invention provides a methodfor determining color using a CMOS image sensor. The CMOS image sensorincludes an n-type substrate and a p-type layer, the p-type layeroverlaying the n-type substrate. The method includes a step for applyinga first voltage on the n-type substrate. The method also includes a stepfor obtaining a first output, which is associated with the firstvoltage. The method additionally includes a step for applying a secondvoltage on the n-type substrate. Additionally, the method includes astep for obtaining a second output, which is associated with the firstvoltage. In addition, the method includes a step for applying a thirdvoltage on the n-type substrate. The method additionally includes a stepfor obtaining a third output, which is associated with the firstvoltage. The method also includes a step for providing a plurality ofweighting factors. The method includes determining a color based on theplurality of weighting factors, the first output, the second output, andthe third output.

According to an alternative embodiment, the present invention provides amethod for forming a CMOS image sensing pixel, which is configured fordetermining a color. The method includes a step for providing an n-typesubstrate that includes a first thickness and a first width. The methodalso includes a step for forming a p-type layer, the p-type layeroverlaying the n-type substrate. The p-type layer includes a secondthickness and a second width. The second thickness and the second widthare associated with a light characteristic. The method additionallyincludes a step for forming an n-type layer, the n-type layer overlayingthe p-type layer. The n-type layer includes a third thickness and athird width. In addition, the method includes a step for forming a pnjunction between the p-type layer and the n-type layer. The pn junctionincludes a fourth width. The method also includes a step for providing acontrol circuit. The control circuit is electrically coupled to then-type substrate.

According to yet another embodiment, the present invention provides animaging capturing device. The image capturing devices includes a userinterface being configured to facilitate an image capturing process. Theimage capture device also includes a first input being configured toreceive a user input. The user input is a command for capturing animage. The image capturing device additionally includes an image sensorbeing configured to capture an image. Additionally, the image capturingdevice includes an optical device being positioned to provide a light toform an image on the image sensor. The image capturing device furtherincludes a processor being configured to process images. The imagesensor includes an n-type substrate, which includes a first width and afirst thickness. The image sensor also includes a p-type layer overlyingthe n-type substrate. The p-type layer includes a second width and asecond thickness. The second width is associated with one or morecharacteristics of a colored light. The image sensor also includes ann-type layer overlying the p-type layer. The n-type layer is associatedwith a third width and a third thickness. The image sensor furtherincludes a pn junction formed between the p-type layer and the n-typelayer.

It is to be appreciated that the present invention provides variousadvantages over conventional technologies. As compared to color filtertechniques. The present invention provides a method and system forcapturing images with true color and pixels. Compared to stackedjunction techniques, the present invention is more cost effective andoffers better imaging capability. For example, the present inventionallows costs savings from the color filtering process and minimumcircuit design. According to an embodiment, only three transistors areneeded for implement an imaging pixel.

Depending upon embodiment, one or more of these benefits may beachieved. These benefits and various additional objects, features andadvantages of the present invention can be fully appreciated withreference to the detailed description and accompanying drawings thatfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a Bayer pattern mask filter used to produce color foran image sensor.

FIG. 2A is a simplified diagram illustrating a conventional image sensorimplemented with stack junction technology.

FIG. 2B is a graph illustrating the absorption coefficient associatedwith color.

FIG. 3 is a simplified diagram illustrating the diffusion principleutilized in conventional CMOS image sensors.

FIG. 4 is a simplified diagram illustrating a working principle of aCMOS image sensor according to an embodiment of the present invention.

FIG. 5 is a simplified diagram illustrating the relationship betweenquantum efficiency and thickness according to an embodiment of thepresent invention.

FIG. 6 is a simplified diagram illustrating the relationship betweenvoltage level and integration time according to an embodiment of thepresent invention.

FIG. 7 is a simplified diagram illustrating an image sensor according toan embodiment of the present invention.

FIG. 8 is a simplified diagram illustrating an image sensor circuitaccording to an embodiment of the present invention.

FIG. 9 is a simplified diagram illustrating an image sensor circuitaccording to an embodiment of the present invention.

FIG. 10 is a simplified diagram illustrating an image sensing pixeldevice having a first photodiode, a second photodiode, and a thirdphotodiode according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to imaging techniques. More particularly, theinvention provides a method and system for capturing images on CMOScolor sensors. Merely by way of example, the invention has been used tocapture true color information on each pixel of an N-type substrate CMOSsensor. But it would be recognized that the invention has a much broaderrange of applicability.

As discussed above, various techniques have been developed to providecolor separation for CMOS based image sensors. Conventional techniquesinvolved using color filter, stack junction photo diode, and junctionseparation. However, convention techniques are often inadequate.

Separating color using color filter has been a popular technique. Thistechnique is based on the concept of additive properties of threeprimary colors red, green and blue (RGB). When the three primary colorscombine, they are capable of producing any other colors. To use colorfilter technique, a color filter is used to produce colors. FIG. 1illustrates a Bayer pattern mask filter used to produce color for animage sensor. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. The Bayermask 100 is divided into many pixels to allow light with certainwavelength to pass down to specific pixel. For example, a group 105 offour pixel filters includes three color filters arranged into a square,which includes a red filter 101, a green filter 102, a blue filter 103,and a red filter 104. Each underlying pixel under the color filtersessentially captures only the light level associated with thatparticular color. The final image that is eventually formed is theresult of calculation and interpolation of value of the pixel and itssurround pixels. For example, the red signal from the pixel underlyingthe red filter 101 is the average of surrounding green pixels. However,sine the color of each pixel is a result of calculation andinterpolation instead of actual color that is directed to the particularpixel, the true color value for that pixel is not obtained. As a result,the color of a captured image may be off, and sometimes the filteringand interpolations produce undesirable artifacts.

To produce “true” color on each pixel, other color separation techniqueshave been developed. For example, stack junction technology is sometimesused to provide color separation. FIG. 2A is a simplified diagramillustrating conventional a image sensor implemented with stack junctiontechnology. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. The stackjunction image sensor 200 includes three color layers. A blue diode 202overlays a green diode 204, which overlays a red diode 206. Each diodeis underneath an n-type region so that pn junctions are formed. Duringan image sensing and color separation process, photon or light passesthrough the blue diode 202 first, the green diode 204 next, and finallythe red diode 206.

The structure of the stack junction image sensor 200 is relatively morecomplex compared that of images sensors using color filtering.Consequently, it is often more expensive and difficult to produce stackjunction image sensors. For example, the manufacturing of stack junctionimage sensor requires additionally epitaxy processes to form colorlayers and additionally structures to make connection and isolation. Inaddition, stack junction image sensors are often vulnerable to noise dueto limited junction capacitance. For example, junction expansion alongin silicon junction has little effect in changing the collectingefficiency since the life-time of minority carrier is usually long insilicon.

The principle of stack junction technology is based on the absorptiondecays associated with silicon depth governed by absorption coefficient.The higher the absorption coefficient, the faster it decays. Forexample, blue photons are associated with higher absorption coefficientand therefore decays faster than green and red photos. FIG. 2B is agraph illustrating the absorption coefficient associated with color.This diagram is merely an example, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. As show in FIG. 2B,the blue photon flux curve 212 drops the most as the silicon depthincreases. And since green has an absorption coefficient that is betweenabsorption coefficients of blue and red, the green photon flux curve 211is between the blue photon flux curve 212 and the red photon flux curve210.

As analyzed above, conventional techniques for image sensing are ofteninadequate. Therefore, it is desirable to have an improved colorseparation scheme.

It is to be appreciated that the present invention utilizes a noveloperating principle as compared to conventional technologies. Forexample, conventional CMOS sensor is implemented with the diffusionprinciple. FIG. 3 is a simplified diagram illustrating the diffusionprinciple utilized in conventional CMOS image sensors. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As illustrated according toFIG. 3, photodiode used in CMOS image sensor a np junction is formed andelectrons are collected by electric field and diffusion. However,silicon is a usually associated with poor photon absorptioncharacteristics. As a result, most carriers are generated far deeper inthe depletion region, especially the red light.

In contrast, the present invention operates, among other things, under adifferent principle. FIG. 4 is a simplified diagram illustrating aworking principle of a CMOS image sensor according to an embodiment ofthe present invention. This diagram is merely an example, which shouldnot unduly limit the scope of the claims. One of ordinary skill in theart would recognize many variations, alternatives, and modifications.According to an embodiment, the present invention utilizes two depletionregions 420 and 440 as illustrated in FIG. 4. An image sensor 400includes an n-type region 410, a depletion region 420, a p substrateneutral region 430, a depletion region 440, and a n-type substrate 450.As compared to the conventional techniques, an np junction between then-type region 410 and the p substrate neutral region 430 is formed. Forthe image sensor 400 to work properly, the reverse bias of the n-typesubstrate 450 to the p substrate neutral region 430 can be changed toadjust the width of the depletion region 440. When the n-type substrate450 is reverse biased, photon-generated carriers beyond the p substrateneutral region 430 can not be collected by the photodiode in the front,therefore, contribute no extra voltage. By changing the width of thedepletion region 440, the quantum efficiency of red or green light canbe adjusted, since they are absorbed deeper in the silicon toward then-type substrate 450.

FIG. 5 is a simplified diagram illustrating the relationship betweenquantum efficiency and thickness according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Asillustrated in FIG. 5, green usually carries the highest quantumefficient, while red generally carries the lowest quantum efficiency. Inaddition, quantum efficiency is directly proportional to the thickness.For example, the quantum efficiency increases as the thicknessincreases.

FIG. 6 is a simplified diagram illustrating the relationship betweenvoltage level and integration time according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications.

FIG. 7 is a simplified diagram illustrating an image sensor according toan embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The image sensor 700 includes a n-type substrate 710at the bottom. For example, the n-type substrate is made of silicon. Ontop of the n-type substrate 710 overlays a p-type epitaxy layer 720.According to an embodiment, the p-type epitaxy layer 720 is formed withadjustable doping concentration and thickness. As an example, the p-typeepitaxy layer 720 consists of p-type silicon. For example, the p-typeepitaxy layer 720 has a thickness ranging from 2 um to 7 um. On top ofthe p-type epitaxy layer 720 overlays an n-type layer 230 to so that apn junction between the p-type epitaxy layer 720 and the n-type layer230 is formed. For example, the n-type layer 230 has a thickness of lessthan 0.5 um. According to certain embodiments, an epitaxy layer 740 isformed between the n-type substrate 710 and the p-type epitaxy layer720. For example, the epitaxy layer 740 consists of silicon germaniummaterial. Depending upon application, the silicon germanium material isused to enhance the red light absorption. For example, because theabsorption coefficient of silicon for red light is poor, and sometimesimplantation of thick p-type epitaxy device could lead to high voltage,the silicon germanium material is added to boost photon absorption. Asmerely an example, a silicon germanium epitaxy layer has a thicknessranges from 0.1 um to 1 um depending on germanium concentration.

According to an embodiment, the present invention applies different biasvoltages on the n-type substrate 710 to obtain different colors. Biasvoltages are based on the light absorption properties of RGB color asexplained above. According to an embodiment, high bias voltage is usedfor blue light, medium bias voltage is used for blue and green light,and zero voltage bias voltage is used for blue, green, and red light.

The operation of an embodiment of the present invention may be describein three steps. At the first step, the p-type epitaxy layer 720 regionis short. As a result, the only carriers collected are those generatedin deletion region formed between the p-type epitaxy layer 720 and then-type layer 730. During the first step, the voltage response from theimage sensor is primarily due to the blue light. The voltage response atthe first step may be expressed according to the following equation.ΔVresponse=BΔVb+g1ΔVg+r1ΔVr  (Equation 1)

According to Equation 1, B is the weighting factor for blue color. Theterm ΔVb represents the response due to the blue color. The term g1ΔVgrepresents the voltage response due to green color. The term r1ΔVrrepresents the voltage response due to red color.

In the second step, a bias voltage that is associated with green coloris applied. The voltage response at the first step may be expressedaccording to the following equation.ΔVresponse=BΔVb+b2ΔVb1+GΔVg+g1ΔVg+(r1+r2)ΔVr  (Equation 2)

According to Equation 2, B is the weighting factor for blue color. G isthe weighting factor for green color. The term BΔVb represents theresponse due to the blue color. The term GΔVg represents the responsedue to the green color. The term g2ΔVg represents the voltage responsedue to green color. The term r2ΔVr represents the voltage response dueto red color. It is noted that green color at the second stepcontributes greatly to the voltage response. Furthermore, the voltageresponse for the green color may be obtained by subtracting the Equation2 from Equation 1.

In the third step, a bias voltage that is associated with red color isapplied. For example, the bias voltage for red color is zero. Thevoltage response at the first step may be expressed according to thefollowing equation.ΔVresponse=BΔVb+b3ΔVb1+GΔVg+(g1+g3)ΔVg+(r1+r2)ΔVr+RΔVr  (Equation 3)

According to Equation 2, B is the weighting factor for blue color. G isthe weighting factor for green color. R is the weighting factor for redcolor. The term BΔVb represents the response due to the blue color. Theterm GΔVg represents the response due to the green color. The term RΔVrrepresents the response due to the red color. The term g2ΔVg representsthe voltage response due to green color. The term r2ΔVr represents thevoltage response due to red color. It is noted that green color at thesecond step contributes greatly to the voltage response. Furthermore,the voltage response for the green color may be obtained by subtractingthe Equation 3 from Equation 2.

It is to be appreciated that the weighting factors B, G, R can bedetermined and calibrated for specific image sensors. According tovarious embodiments, the present invention provide color separationschemes with fixed photodiode depletion, which could simplify thecircuit design and process. FIG. 8 is a simplified diagram illustratingan image sensor circuit according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. Asillustrated according to FIG. 8, three transistors 830, 840, and 850 areused for blue, green, and red color based on voltage.

FIG. 9 is a simplified diagram illustrating an image sensor circuitaccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. The CMOS sensor unit 10includes a photo detector 13, a source follow transistor 15, a rowselect transistor 16, and a CMOS circuit 18. As merely an example, thetransistors in the CMOS sensor unit 10 control the on and off states toread light signal at proper time. According to an embodiment, typicaloperation involves three steps, each step is for capture an individualcolor: blue, green, or red. The back side is a reverse biased pnjunction. When doing blue light absorption, a high voltage is used, sothat red and green photons will not be collected. When doing green lightabsorption, a reduced bias is used and green photons will be added tothe response. Finally, all the back side bias is removed to collect all.As merely an example, the back side bias is synchronized with the mainlycircuit signal process timing.

According to an embodiment, the present invention provides a CMOS imagesensing pixel. The pixel includes an n-type substrate, which includes afirst width and a first thickness. The pixel also includes a p-typeepitaxy layer overlying the n-type substrate. The p-type epitaxy layerincludes a second width and a second thickness. The second width isassociated with one or more characteristics of a colored light. Thepixel additionally includes an n-type layer overlying the p-type epitaxylayer. The n-type layer is associated with a third width and a thirdthickness. Additionally, the pixel includes an pn junction formedbetween the p-type epitaxy layer and the n-type layer. Moreover, thepixel includes a control circuit being coupled to the CMOS image sensingpixel.

According to another embodiment, the present invention provides a methodfor determining color using a CMOS image sensor. The CMOS image sensorincludes an n-type substrate and a p-type layer, the p-type layeroverlaying the n-type substrate. The method includes a step for applyinga first voltage on the n-type substrate. The method also includes a stepfor obtaining a first output, which is associated with the firstvoltage. The method additionally includes a step for applying a secondvoltage on the n-type substrate. Additionally, the method includes astep for obtaining a second output, which is associated with the firstvoltage. In addition, the method includes a step for applying a thirdvoltage on the n-type substrate. The method additionally includes a stepfor obtaining a third output, which is associated with the firstvoltage. The method also includes a step for providing a plurality ofweighting factors. The method includes determining a color based on theplurality of weighting factors, the first output, the second output, andthe third output. For example, the embodiment is illustrated accordingto FIG. 7.

According to an alternative embodiment, the present invention provides amethod for forming a CMOS image sensing pixel, which is configured fordetermining a color. The method includes a step for providing an n-typesubstrate that includes a first thickness and a first width. The methodalso includes a step for forming a p-type layer, the p-type layeroverlaying the n-type substrate. The p-type layer includes a secondthickness and a second width. The second thickness and the second widthare associated with a light characteristic. The method additionallyincludes a step for forming an n-type layer, the n-type layer overlayingthe p-type layer. The n-type layer includes a third thickness and athird width. In addition, the method includes a step for forming a pnjunction between the p-type layer and the n-type layer. The pn junctionincludes a fourth width. The method also includes a step for providing acontrol circuit. The control circuit is electrically coupled to then-type substrate. For example, the embodiment is illustrated accordingto FIG. 7.

According to yet another embodiment, the present invention provides animaging capturing device. The image capturing devices includes a userinterface being configured to facilitate an image capturing process. Theimage capture device also includes a first input being configured toreceive a user input. The user input is a command for capturing animage. The image capturing device additionally includes an image sensorbeing configured to capture an image. Additionally, the image capturingdevice includes an optical device being positioned to provide a light toform an image on the image sensor. The image capturing device furtherincludes a processor being configured to process images. The imagesensor includes an n-type substrate, which includes a first width and afirst thickness. The image sensor also includes a p-type layer overlyingthe n-type substrate. The p-type layer includes a second width and asecond thickness. The second width is associated with one or morecharacteristics of a colored light. The image sensor also includes ann-type layer overlying the p-type layer. The n-type layer is associatedwith a third width and a third thickness. The image sensor furtherincludes an pn junction formed between the p-type layer and the n-typelayer. For example, the embodiment is illustrated according to FIG. 7.

It is to be appreciated that the present invention offers an improvementover both the color filtering technique and stack junction techniques inmany ways. For example, integrating color separation capability into onepixel has great advantage over using several pixels with color filter.Capturing true color with each individual pixel has better colorresolution and result in better image quality. Thus stack junctiontechnique often provide better image quality. However, to accomplishthis resolution, the stack junction technique usually sacrifice area andcost. Typically, the stack junction technique requires three moretransistors to individually control blue, green and red photodiode. Suchhardware requirement diminishes the ability to shrink the pixel sizecompared to the conventional three transistor design. For example, theconnecting plugs and contacts for green and red diode occupy valuablepixel area. As a result, pixel becomes difficult to shrink beyondcertain size. Moreover, the extra two epitaxy layers not only add cost,but also bring difficulty to control yield and uniformity during themanufacturing process. More transistors make the circuitry even moredifficult to design and process. For example, the embodiment isillustrated according to FIGS. 7 and 9.

Among other things, the present invention provides a better colorseparation capability and lower noise level in the image capturingprocess. Often, the color separation capability is intrinsiccharacteristic associated with long diffusion length. Generally, diodequantum efficiency has very little dependency on the depletion width.

Noise level is often associated with junction capacitance. For example,junction capacitance is inversely related to the reverse bias voltage.Usually, the capacitance is described according to the followingequation:V=1/C ²  (Equation 4)

As illustration by Equation 4, small capacitance leads to large ΔV sinceΔV=ΔQ/C. Even with fixed noise level ΔQ, AV will be larger. Typically ΔQincreases as the reverse bias increases.

The present invention provides improved and better solution in terms ofcolor separation capability and lower noise level. The reversed biaseddepletion region between the n-type substrate and the p-type layer actsas a valve that controls the photon generated carriers flowing into theactive pixel region. For example, the present invention allows changingthe photodiode quantum efficiency significantly by shrinking orexpanding the width of the p-type layer. According to variousembodiments, the present invention does not require adjusting biasvoltage of the active pixel or the capacitance change. As a result,noise level can be kept constant.

It is understood the examples and embodiments described herein are forillustrative purposes only and that various modifications or changes inlight thereof will be suggested to persons skilled in the art and are tobe included within the spirit and purview of this application and scopeof the appended claims.

1. A CMOS image sensing pixel comprising: an n-type substrate, then-type substrate including a first width and a first thickness; a p-typeepitaxy layer overlying the n-type substrate, the p-type epitaxy layerincluding a second width and a second thickness, the second widthincluding an adjustable depletion region between the n-type substrateand the p-type epitaxy layer, the adjustable depletion region beingassociated with one or more characteristics of a colored light; ann-type layer overlying the p-type epitaxy layer, the n-type layer beingassociated with a third width and a third thickness; a pn junctionformed between the p-type epitaxy layer and the n-type layer; and acontrol circuit being coupled to the CMOS image sensing pixel.
 2. TheCMOS image sensing pixel of claim 1 wherein the p-type epitaxy layerincludes a silicon material.
 3. The CMOS image sensing pixel of claim 1wherein the p-type epitaxy layer has a thickness range of 2 um to 7 um.4. The CMOS image sensing pixel of claim 1 wherein the control circuitincludes a color selecting component.
 5. The CMOS image sensing pixel ofclaim 1 wherein the n-type substrate includes a silicon material.
 6. TheCMOS image sensing pixel of claim 1 wherein the second width isassociated with a quantum efficiency.
 7. The CMOS image sensing pixel ofclaim 1 wherein the second width is associated with a quantumefficiency, the quantum efficiency being associated with a color.
 8. TheCMOS image sensing pixel of claim 1 wherein the third width isassociated with a depletion region caused by the n-type layer and thep-type epitaxy layer.
 9. The CMOS image sensing pixel of claim 1 whereinthe control circuit comprises a voltage source for applying a firstvoltage, a second voltage, and a third voltage to the n-type substrate,wherein: the first voltage is configured for resulting in a firstoutput, the first output being associated with a first color; the secondvoltage is configured for resulting in a second output, the secondoutput being associated with a second color, and the third voltage isconfigured for resulting in a third output, the third output beingassociated with a third color; wherein the first color, the secondcolor, and the third color are further determined using a firstweighting factor, a second weighting factor, and a third weightingfactor.
 10. The CMOS image sensing pixel of claim 1 further comprising afirst epitaxy layer, the first epitaxy layer being positioned betweenthe n-type substrate and the p-type silicon epitaxy layer.
 11. The CMOSimage sensing pixel of claim 10 wherein the first epitaxy layer includesa silicon germanium material.
 12. An image sensing pixel, comprising: ann-type silicon substrate, the n-type semiconductor substrate including afirst width; a p-type silicon layer overlying the n-type substrate, thep-type silicon layer including a second width; an n-type silicon layeroverlying the p-type silicon layer, the n-type silicon layer beingassociated with a third width; a pn junction formed between the p-typesilicon layer and the n-type silicon layer; and a silicon germaniumlayer in the p-type silicon layer, the silicon germanium layer beingpositioned away from the pn junction.
 13. The image sensing pixel ofclaim 12 wherein the silicon germanium layer is free from pn junctions.14. The image sensing pixel of claim 12 wherein the silicon germaniumlayer is configured for absorption of red light.
 15. An image sensingpixel device, comprising: a first photodiode having a first p-typeregion and a first n-type region forming a first pn junction overlying afirst n-type substrate; a second photodiode having a second p-typeregion and a second n-type region forming a second pn junction overlyinga second n-type substrate; a third photodiode having a third p-typeregion and a third n-type region forming a third pn junction overlying athird n-type substrate; a terminal coupled to the first n-type region,the second n-type region, and the third n-type region; a first biasvoltage coupled to the first n-type substrate; a second bias voltagecoupled to the second n-type substrate; and a third bias voltage coupledto the third n-type substrate.
 16. The image sensing pixel device ofclaim 15, wherein: the third p-type region further comprising a silicongermanium region disposed away from the third pn junction.
 17. The imagesensing pixel device of claim 15, wherein the first bias voltage isconfigured for absorption of blue light.
 18. The image sensing pixeldevice of claim 15, wherein the second bias voltage is configured forabsorption of green light.
 19. The image sensing pixel device of claim15, wherein the third bias voltage is configured for absorption of redlight.